Certain machinery components emit sounds when operating under normal conditions. These sounds can be considered to be annoying, but can be reassuring as a sign that the machinery component is operating normally. For example, personal computers are equipped with fans that keep the CPU cool during operation. Although computer fans have become more quiet, the blowing sound made by the fan, or the hum emitted by the fan motor, is usually discernible. The absence of this sound, or a change in its usual qualities, can be interpreted as a potential problem in the functioning of the computer.
Other types of machinery perform continuously over certain periods of time, and emit characteristic sounds the entire time of performance. If the machinery component normally emits a distinct sound at regular intervals, the sudden absence of this sound could be a sign that the machinery component is experiencing degradation in performance, or even failure. A person monitoring the function of the machinery component could recognize the absence of the sound and investigate the cause of the interruption. However, the person might be distracted and wouldn't necessarily notice the absence of the sound. Further, a change in pitch or frequency, indicating degradation, might not be easily recognized. Also, some machinery operates unattended for at least some periods of time, and therefore the absence of the sound would not be noticed.
For example, one type of magnet, used in NMR imaging systems and for other applications, is ouffitted with superconducting magnet coils. These coils must be kept below a particular temperature in order to function properly. For example, the magnet coils can be maintained within a selected operating temperature range by using liquid helium, which is kept cool by cryocoolers, typically one for each magnet coil. The compressor for the cryocoolers can be air cooled or water cooled. If the compressor is water cooled, and if access to the cooling water is accidentally cut off, such as if water access is turned off in the building or if water pressure drops due to an emergency, the cryocoolers can go into thermal shutdown. Even after the water is turned back on, the cryocoolers typically need to be manually restarted and do not turn themselves back on. Without the cryocoolers running, the magnet will not work properly.
A cryocooler makes a regular chirping sound under normal operating conditions. Under conditions that cause performance of the cryocooler to degrade or fail, this chirp can become less regular in its rate and pitch, can occur at less frequent intervals, or can even stop, depending on the particular malfunction. It would be advantageous to provide a process by which a change or absence of this chirp would be recognized, and an indication, such as an alarm, provided as notification of the change. It would also be advantageous to provide an apparatus that can perform such a process.